1. Field of the Invention
The present invention generally relates to devices for measuring fluid properties, and also generally relates to oil and gas well (borehole) logging tools. More particularly, the invention relates to an improved method and apparatus for determining the density of drilling fluid by measuring the acoustic impedance and the sonic velocity of the fluid in a borehole. The same method and apparatus can be used to locate and determine the quality of a downhole wet cement abandonment plug positioned in a borehole.
2. Description of Related Art
In various industrial processes that involve fluid material, it is useful to know the properties of the fluids involved. These fluid properties include, for example, density, compressibility, reflectance, acoustic impedance, viscosity and attenuation. Knowledge of the values of these various properties can be used to adjust process parameters or warn of impending calamity. In many applications, such as oil and gas well (borehole) drilling, fluid density is of particular interest. It is important to know the density of drilling fluid (also referred to as drilling mud) during a drilling operation, in order to prevent a blowout of the well.
In a drilling operation, drilling fluid is pumped down the drill string (essentially a very long pipe), exits at the drill bit, and then returns to the surface within an annulus formed between the outside of the pipe and the inside of the borehole. As the bit drills into the geologic formations, it passes through zones containing various fluids, including lightweight fluids such as saltwater, oil (hydrocarbons), and natural gas. If the pressure within the zone is greater than the pressure within the borehole, these fluids will enter the borehole and mix with the drilling fluid. When the aforementioned lightweight fluids mix with drilling fluid, its density decreases. If the total weight of fluid within the borehole decreases too much, it can lead to a blowout when a high-pressure zone is entered. It is therefore very important that the density of the drilling fluid be accurately monitored. In producing wells the fluid density, with other measurements, is used to infer the proportions of oil, water and natural gas that the well is producing at various depths in the well. Logging tools for measuring fluid density are well known.
One common prior-art technique for measuring drilling fluid density involves the use of acoustic transducers, particularly ultrasonic transducers, as described in U.S. Pat. No. 4,571,693. That device uses an ultrasonic transducer coupled to the body of a probe to transmit and receive a signal across a first solid/fluid interface and a second fluid/solid interface, in order to measure the sound velocity of the fluid. A second signal is a reference signal generated by reflection off a surface that is hermetically sealed from contact with the fluid. Measurement of the signals reflected off the two surfaces are used to calculate reflectance and acoustic impedance, from which density may be inferred.
One problem encountered with the foregoing approach is that an ultrasonic transducer can lose the acoustic coupling, that is, the ability to transfer the acoustic energy, when in poor contact with the body of the tool, which is typically built of a metal material such as steel. It only requires a very small gap (in the thousands of an inch) to lose nearly 100% of the transmitted energy, since a vacuum does not transmit any sound.
Another problem encountered in such prior art fluid density measurement techniques is that, during the measurement of the velocity of sound in the fluid, the signal must pass through two solid/fluid and fluid/solid interface-transmissions, plus one fluid/solid interface-reflection. Since the acoustic impedance difference between metals and fluids is on the order of 30 to 1, only about 2% of the transmitted signal is ever received back. This loss of signal does not take into consideration the further attenuation suffered during propagation of the signal in the fluid and the metal.
The foregoing prior-art method clearly cannot be used to measure the properties of heavy drill fluids, oil-based drill fluids, or wet cement abandonment plugs, where the signal attenuation at ultrasonic frequencies is very high, well above 20 dB/inch attenuation rate. In oil and gas producing areas, it is often necessary to permanently isolate different strata by placing cement plugs at selected locations along the borehole. A cement abandonment plug is placed in the open hole by pumping a special mixture of water and cement down the drill pipe, displacing the drilling mud within the pipe and the surrounding area of the borehole. The drill pipe is then raised until it is above the wet plug. After placement of the wet plug in the borehole, the location of the top of the plug must be determined to ensure that the plug has the required size. Prior art techniques for locating and determining the quality of downhole wet cement abandonment plugs, such as that described in U.S. Pat. No. 5,036,916, do not, however, locate both the top and bottom of the cement plug. That method in particular is invasive (taking a sample of the cement for further analysis at the surface), and very time-consuming in operating the sample chamber to draw cement into the chamber, costing rig time, and incurring the associated risk and expense.
Other prior-art methods and apparatuses for measuring the fluid density in boreholes, such as those described in U.S. Pat. Nos. 4,939,362 and 5,204,529, include the use of either chemical radio-active sources or electrically-activated radioactive sources, which present clear environmental and health hazards. It is therefore apparent that a need exists for an improved acoustic well logging tool and method to determine the density of fluid in boreholes. It would be further advantageous if the tool and method included the detection and the determination of the quality and location of wet cement abandonment plugs.